Adds WAIST flag for clothing.

A new clothing layer for belts and anything similar worn around or on
your waist, which allows you to comfortably wear a belt along with your
backpack. Sheathes and holsters have been given the tag, but left on the
LEGS slot since they can theoretically interfere with movement there and
also so that they can be worn with a belt without directly interfering
with it.

Affected: tool belt, firefighter belt, fanny pack, tactical dump pouch,
leather belt, holster, sheath, scabbard, rag pouch, leather pouch,
survivor utility belt

Grants locations and coverage to wearable containers, which they
were missing.

Affected: both canteens, all three waterskins

Includes documentation for new item flag.

- closes https://github.com/CleverRaven/Cataclysm-DDA/issues/10277